Say you have a normal pneumatic launcher, lets assume a barrel sealing piston valve for now and pressurize it with a propane-air mixture instead of just air.
Then, between the sealing face of the piston and the projectile (the dead space), put another fuel mixture, preferably something like acetylene or hydrogen (propane may work too, I dont know).

When the piston valve opens, the air smacks into the dead space fuel mixture, rapidly compressing it. Shock heating -> ignition. As air goes into the barrel, the flamefront goes into the chamber.

To contain the fuel mixture between the projectile and the piston, you could use a balloon to contain it. Simply load the balloon behind the projectile, and you're done.

The main reason of this is not to create extra pressure (which also happens), but to heat up the air, effectively raising the speed of sound.
The SOS of air is the largest barrier in most high pressure, high velocity launchers. Raising it, using either a lighter gas or higher temperatures, will allow the projectile to be accelerated above the SOS without having a ridiculously large launcher.

Your opinions!

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

But today, has anyone tried something like this? Do you think this is worth trying?

Even though acetylene works for sure, propane could probably also do the trick in the dead space. Just make sure you've got enough pressure and a fast opening valve and it will be heated up past its ignition point.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

But today, has anyone tried something like this? Do you think this is worth trying?

Even though acetylene works for sure, propane could probably also do the trick in the dead space. Just make sure you've got enough pressure and a fast opening valve and it will be heated up past its ignition point.

Does anyone know the auto ignition point and the pressure rise needed to reliably exceed it?

I don't think the energy created in the rapid compression would be dense enough to cause an ignition. Much like the problems with spark gaps, the energy needs to be concentrated in a very small area in order to ignite propane, especially with higher mixes.

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"Never tell people how to do things. Tell them what to do and they will surprise you with their ingenuity." George S. Patton

Simplistic models say that nothing should happen: when you increase the total volume, the partial pressure of the acetylene should decrease, regardless of the pressure in the other chamber. Also, the overall temperature of the chamber + dead space should decrease when the piston moves backward.

Despite that, I'm not saying that it wouldn't work - just that I don't know enough about fluid dynamics to model precisely what situations it would work in. I read somewhere here about a theory D_Hall had regarding a localized heating that may occur in the dead space due to shock compression when the main valve (piston/diaphragm or burst disk here) was opened.

"Springer" airguns operate in a completely different manner to this: they change the volume of the container, thereby increasing the partial pressure of all its components. The exact relationship in this case is fairly simple, and can be used to find the volume change needed to ignite the gas. I'm not sure how well (or if) this works for more complex gases like ethyne, but it should be just fine for hydrogen:

Where:
v<sub>2</sub> = the final volume needed
v<sub>1</sub> = the initial volume
a = the number of degrees of freedom in the gas
R = gas constant
n = number of moles of gas
T<sub>1</sub> = initial temperature
T<sub>2</sub> = final temperature
y = adiabatic constant for the gas ( equal to (a + 1)/a)

This may also apply to some extent in the scenario that the original poster presented, but the piston effect would, as far as I know, be greatly reduced because the "piston" can mix with the gas it is supposed to be compressing.

With high speed (read: $$$$, or perhaps $$$$$) equipment and/or some sort of Hydra code based simulator (good luck ) , one could build a pretty good picture of precisely what happens in the dead space, but short of that, such a design would rely, at least to some degree, on trial and error.

Simplistic models say that nothing should happen: when you increase the total volume, the partial pressure of the acetylene should decrease, regardless of the pressure in the other chamber. Also, the overall temperature of the chamber + dead space should decrease when the piston moves backward.

That only counts when things happen slowly (very slowly).
Because the air bumps into the acetylene mix with a speed near the speed of sound, normal thermodynamics do not apply anymore.
The acetylene mix compresses, raising its temperature.
You can imagine the air chamber as a spring instead of an air chamber. The gas simply does not have the time to mix.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

I say that there is only one economical way to determine if this works: try it.

IIRC (from the detonation gun thread) a 1:10 compression is needed for propane. that means that you should theoretically use at least 150psi or 11x in the main chamber. This may not be necessary if the air has sufficient inertia to compress it further.

Note that this should NOT be done with acetylene in copper or brass cannon because acetylene forms primary high explosives with copper. That said, hydrogen has a lower ignition energy, is easier to get/make, and doesn't randomly blow up above around 15psi.

If all else fails, add a spark plug and have yourself a piston hybrid.

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POLAND_SPUD wrote:even if there was no link I'd know it's a bot because of female name

I didn't say that the simplistic ideas were correct, just that they wouldn't work here.

Also, the applicability of "normal thermodynamics" to the situation hinges heavily on one's definition of "normal thermodynamics". If "normal thermodynamics" is what one learns in Grade 10, then no, it does not apply.

What I think the sticking point is, is how much the advancing gas through the opened valve behaves as a piston, and how much it behaves as a slow moving gas. I don't know enough about fluid dynamics to give an answer here, but it may not be as coherent as you think - the body of gas as a whole accelerates, but the individual particles don't, as they were already bouncing around the chamber at 500m/s before you opened the valve. Once again, I'm not saying that it couldn't work, just that it would not be equivalent to a piston made of a solid substance.

Trying it will determine whether or not it works in a particular launcher. Said launcher could be designed by looking at how it would work with a solid piston, and then adding as much room for error caused by the gaseous piston as you saw fit.

Ethyne doesn't seem like the greatest fuel to play with in hybrids, but a launcher that used a piston to detonate pure acetylene would be very interesting. Starting at *almost safe* pure acetylene at atmospheric pressure, very little compression would be required to detonate it (compressing it *very* rapidly to half its original volume would bring it to 24.1 psig, and one quarter its original volume would yield 87.7psig).

KineticAmbitions wrote:I didn't say that the simplistic ideas were correct, just that they wouldn't work here.

Also, the applicability of "normal thermodynamics" to the situation hinges heavily on one's definition of "normal thermodynamics". If "normal thermodynamics" is what one learns in Grade 10, then no, it does not apply.

What I think the sticking point is, is how much the advancing gas through the opened valve behaves as a piston, and how much it behaves as a slow moving gas. I don't know enough about fluid dynamics to give an answer here, but it may not be as coherent as you think - the body of gas as a whole accelerates, but the individual particles don't, as they were already bouncing around the chamber at 500m/s before you opened the valve. Once again, I'm not saying that it couldn't work, just that it would not be equivalent to a piston made of a solid substance.

Trying it will determine whether or not it works in a particular launcher. Said launcher could be designed by looking at how it would work with a solid piston, and then adding as much room for error caused by the gaseous piston as you saw fit.

Ethyne doesn't seem like the greatest fuel to play with in hybrids, but a launcher that used a piston to detonate pure acetylene would be very interesting. Starting at *almost safe* pure acetylene at atmospheric pressure, very little compression would be required to detonate it (compressing it *very* rapidly to half its original volume would bring it to 24.1 psig, and one quarter its original volume would yield 87.7psig).

Interesting thought. I'm wondering if a gas equivilant of the WWII multi stage supergun could be built to compensate for the cooling of gasses by expansion.

The V-3 (Vergeltungswaffe 3), also known as the Hochdruckpumpe ("High Pressure Pump", HDP for short) and Fleissiges Lieschen ("Busy Lizzie"), was a German World War II supergun working on the multi-charge principle whereby secondary charges are detonated to add velocity to a projectile.

A multistage piston/burst disk/projectile and multi charge is an interesting concept if using compression to set of the chages would be a very interesting project. It would need someone with better skill than me to engineer it.

IIRC (from the detonation gun thread) a 1:10 compression is needed for propane. that means that you should theoretically use at least 150psi or 11x in the main chamber. This may not be necessary if the air has sufficient inertia to compress it further.

Only 150 psi in the main chamber? Thats a good thing. I was thinking to use up to 450.
Now since my primary reason to get this combustion started is to raise SOS instead of increasing pressure, I may put a very lean gas mixture in the main chamber, just enough to deflagrate.

Note that if the gas in the dead space is contained inside a balloon, then there's your piston.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!